As various electronic equipment becomes smaller in size and higher in function, the demand for higher mounting density of electronic elements has been increased. To meet this demand, a multilayer wiring board in which an insulating layer and a wiring layer are stacked alternately has been widely used. To meet the demands for high mounting density and high performance, the wiring board having multiple wiring layers and each wiring layer is connected by via connection such as a via hole to the other wiring layer.
FIG. 24 is a sectional view showing one example of a construction of conventional multilayer wiring board. This multilayer wiring board 901 is constructed by via connecting wiring circuits formed in five layers. A first wiring layer 901, a second wiring layer 902, a third wiring layer 903, a fourth wiring layer 904, and a fifth wiring layer 905 are each formed by patterning a conductor layer. These wiring circuits are isolated by an insulating layer 906.
A general method for manufacturing a multilayer wiring board having a typical structure as shown in FIG. 24 will be described below.
First, in order to connect the layers of a double faced laminate, in which a conductor layer such as copper foil is bonded to both faces of insulating layer, through holes 907 are formed at portions to be electrically connected of the double faced laminate. Chemical plating is applied to the inside wall surface of the through hole 907, and further electroplating is applied to increase the thickness of a conductor layer 907b on the inside wall surface of the through hole 907, thereby enhancing the reliability of connection between layers.
Then, the conductor layers on both faces are patterned into a predetermined circuit by, for example, the photo-etching process.
Next, an insulating layer such as a prepreg layer is laminated on the patterned conductor layer and further a conductor layer such as copper foil is laminated to achieve integration by heating and pressurization. By repeating the process from the formation of through holes to the patterning of circuit, a multilayer wiring board is formed.
Such a multilayer wiring board, in which via hole is used for connection between wiring layers, has a problem in that it is difficult to respond to high-density mounting of a electronic element.
For example, generally speaking, neither wiring is formed nor electronic elements are mounted in the region where a through hole is provided, so that the increase in wiring density and mounting density is restricted. Also, in recent years, along with high-density mounting of electronic elements, the wiring of wiring board has become dense. If attempts are made to decrease the diameter of through hole in response to high density of wiring, it becomes difficult to assure reliability of connection between layers.
The connection between wiring layers using through holes is redundant because it entails a process for forming through holes and a process for plating, which presents a problem of productivity.
For example, the process for forming through holes requires much time for drilling work because holes are made one after another by using a drill. Also, the position where a through hole is formed requires a high accuracy, and the plating adhesion property etc. on the inside wall surface of through hole must be considered. Therefore, the control of accuracy of through hole formation, forming conditions, etc is troublesome.
In addition, in the plating process where electrical connection between a plurality of wiring layers is formed via through holes, process control such as control of concentration and temperature of chemicals is also troublesome. Further, large-scale equipment is required for forming through holes and plating.
Such connection between layers of multilayer wiring board using through holes decreases the productivity of wiring board (PWB), so that it is difficult to meet the demand for lower cost.
To simplify the electrical connection between wiring layers of multilayer wiring board, a method in which the connection between wiring layers is performed by means of conductive bumps has also been proposed. With this method, conductive bumps are formed on a via land, which is an interlayer connection formed in the wiring circuit, and by inserting an interlayer insulating layer in the thickness direction, the connection with the via land formed on the opposite wiring layer is established.
FIG. 25A and FIG. 25B show one example of a method for manufacturing a multilayer wiring board in which the wiring layers are connected by using such conductive bumps.
First, a double faced wiring board 913, in which wiring circuit 912 consisting of copper is formed on both surfaces of, for example, paper/phenol base insulating resin substrate 911, is prepared as an core layer. The wiring circuit 912 formed on both surfaces of the insulating resin substrate 911 has a via land 912a for interlayer connection. On this via land 912a, a conductive bump 914 formed by printing, for example, conductive paste is formed.
Next, an insulating resin sheet 915 of B stage (semi-cured) and a copper foil 916 are laminated, and on both sides of the double faced wiring board 913, the wiring circuit 912 and the copper foil 916 are arranged via the insulating resin sheet 915 so as to face each other (FIG. 25A).
By pressurizing and heating these laminates, the insulating resin sheet 915 of B stage is cured to integrate all layers. At this time, the conductive bumps 914 pierce the insulating resin sheet 915 of B stage (semi cured) by pressure, and are bonded in integration with the copper foil 916 while producing plastic deformation etc. Thus, the connection between conductor layers is formed by the conductive bumps.
A through hole 917 is formed at a predetermined position, and the through hole 917 is filled with a conductive material such as silver paste 918, or the inside wall of the through hole 917 is coated with a conductive material such as silver paste, by which the conductor layers of outer layer are connected to each other. The copper foil 916 of outer layer is patterned by, for example, the photo-etching process to form a predetermined wiring circuit 916b including the via land 916a, by which a multilayer wiring board in which the conductive bumps and through holes are combined for interlayer connection of wiring circuit is formed (FIG. 25A).
FIG. 26A and FIG. 26B show another example of a method for manufacturing a multilayer wiring board in which the wiring layers are connected by using the conductive bumps.
First, a double faced wiring board 923, in which a wiring circuit 922 is formed by affixing, for example, glass cloth and copper foil to both surfaces of an epoxy resin substrate 921, curing, and patterning, is prepared as an inside layer core. The wiring circuit 922 formed on both surfaces of the double faced wiring board 923 has a via land 922a for interlayer connection.
On the other hand, a copper foil 925 formed with conductive bumps 924 and an epoxy resin prepreg 926 are prepared. The conductive bumps 924 are formed at positions such as to correspond to the via land 922a when the copper foil 925 and the double faced wiring board 923 are laminated.
Next, as shown in FIG. 26A, after the copper foil 925 is arranged on both sides of the double faced wiring board 923 via the prepreg 926, all layers are integrated by pressurization and heating. By this pressurization, the conductive bumps 924 face each other. At this time, the conductive bumps 924 pierce the prepreg 926 of B stage (semi cured) by pressure, and are bonded in integration with the via land 922a while producing plastic deformation etc. Thus, the connection between conductor layers is formed by the conductive bumps.
A through hole 927 is formed at a predetermined position, and the through hole 927 is plated with a conductor layer 918 such as copper, for example, by which the conductor layers are connected to each other.
Thereafter, the copper foil 925 of outer layer is patterned by, for example, the photo-etching process to form a predetermined wiring circuit 926b including the via land 925b, by which a multilayer wiring board in which the conductive bumps and plated through holes are combined for interlayer connection of wiring circuit is formed (FIG. 26B).
Such interlayer connection of wiring circuits employing conductive bumps has advantages of simple construction, high productivity due to a small number of processes, and response to high-density mounting. However, the wiring board in which interlayer connection of wiring layer is made by using the conductive bumps often presents the problems as described below.
For example, in the manufacturing method shown in FIG. 25, the insulating resin layer 915 and the copper foil 916 are laminated on the double faced wiring board 913 in which conductive bumps 914 are formed on the via land 912a, a hard metallic press plates are applied from the outside, and all layers are integrated by pressurizing and heating from both surfaces.
By this pressurizing process, the copper foil 916 exposed to the outer layer becomes substantially flat. However, since the conductive bump 914 is generally harder than the insulating resin substrate 911 of the double faced wiring board 913, the via land 912a is liable to become concave, and cracks are produced on the via land 912a. Thus, the via connection is sometimes damaged.
Such a problem is remarkable especially on the connection on outer layer side. FIG. 27 schematically shows a via connection of the MLB shown in FIG. 7 made concave by the pressurizing process. In the worst case, the connection between the via land 922a and the wiring circuit 922 are broken off, so that the function as multilayer wiring board is sometimes lost. The method shown in FIG. 25 also presents the exactly same problem.
Such a problem greatly decreases the reliability of multilayer wiring board, and is one of the main causes for decreasing the productivity of wiring board.
The present invention was made to solve the above problems, and accordingly an object thereof is to provide a multilayer wiring board capable of being constructed by a simple means and having highly reliable via connection. Another object of the present invention is to provide a manufacturing method with high productivity for a highly reliable multilayer wiring board.
As a problem of a multilayer wiring board using a conventional conductive bump, it is desired to more greatly improve reliability of via connection in case of bearing a thermal load or a mechanical load (stress).
The reliability of the wiring board in which interlayer connection of wiring circuit is made by using the conductive bumps, as shown in FIG. 25 and FIG. 26, especially poses a problem under the ordinary operating conditions.
However, when the wiring board using conductive bumps is used for equipment requiring high reliability, such as industrial electronic equipment, the reliability of interlayer connection is feared.
The wiring board requiring high reliability must sufficiently withstand a severe thermal stress. Even when a thermal cycle test, in which the wiring board is exposed to a high temperature of, for example, about 125.degree. C. and then cooled to about -65.degree. C., is performed, the conductive bump for connecting a plurality of wiring layers must not be peeled off, or the wiring circuit in the same layer must not be impaired.
However, the connection between the conductive bump and the wiring layer connected by this conductive bump is sometimes broken off by the thermal stress, so that it is difficult to assure high reliability such as to withstand the industrial application.
The present invention was made to solve such a problem. Accordingly, an object of the present invention is to provide a wiring board having an improved reliability of interlayer connection, and materials for the wiring board, and a method for manufacturing the wiring board.
In addition, since electronic elements including a semiconductor device have been remarkably highly integrated in recent years, problems in mounting have also appeared. For example, the number of connecting points between an electronic element and an external circuit has been increased with the progress of higher integration. It is demanded to establish a high-density mounting technology capable of performing connection between more electrodes in a smaller area. Additionally, since a conventional mounting method using a solder bump or the like uses solder containing lead, there is a problem that workers' health is injured or it is difficult to dispose it as a waste matter.